1. Field of the Invention
The present invention relates generally to a wind or ocean flow passively vented Savonius Rotor assembly.
2. Description of Related Art
Horizontal-axis wind turbines (HAWTS) and Vertical-axis wind turbines (VAWTS) are susceptible to the Betz Limit criteria (i.e., 16/27ths), whereby they lose at least 41% of the theoretical extractable energy from either wind velocity OR water velocity. Thereafter, the energy extraction process is solely dependent on the turbine overall efficiency. The turbine overall efficiency (ηo) consists of blade aerodynamic efficiency (ηb) times the associated mechanical efficiency (ηm) times the electrical conversion process efficiency (ηe) to produce the resultant electrical power. These efficiency terms are combined into an expression to determine the maximum extractable energy in Watts/m2 vs. the wind or current velocity. This can be represented by the following expression:Watts/m2=0.50×(ρ, kg/m3×(wind vel., m/sec.)3×Betz Limit×ηo where ρ=1.225 kg/m3 at sea level elevation and 68° F.or, Watts/m2=0.363×(wind vel., m/sec.)3×(ηb×ηm×ηe)
Current wind turbine industry practice is to measure the output power from their generators without consideration of the power conditioning and conversion processes necessary for establishing grid compatibility. The reported total Watts generated is then simply divided by the rotor swept area to determine the specific energy at that wind velocity. These curves are then used in sales brochures to present documented performance capabilities. Unfortunately, this practice assumes that energy is being uniformly extracted over the entire swept area. This is not the case, as the rotor delivery torque times the rotor RPM is proportional to the input power supplied to the gearbox. The torque is composed of the summation of lift and drag forces acting at varying distances along the blade from the rotor hub to the blade tips. These forces are proportional to the blade rotational velocity2 at any particular distance from the hub. Integration of the resultant torque as a function of incremental distances along the blade will show that ˜90% of the energy extracted is being provided by the outer 30% of the rotor disc. (or ˜50% of the area) This leads to the surprising conclusion that the past practice of using the entire swept area of the rotor disc to estimate the energy extracted must be reduced by half, revealing that reports of blade performance aerodynamic efficiency are ˜2× higher than is actually the case.
It is evident that for conventional wind turbines, wind velocities remain unchanged as they pass through the inner 70% of the rotor disc, causing large flow-field discontinuities downstream. Mixing of the highly disturbed outer flow field with that of the essentially undisturbed inner flow field, generates massively swirling eddies downstream of the rotor.
The blade aerodynamic efficiency ηb is determined by the lift/drag ratio (CL/CD) of the blade. This ratio is usually low, because a sufficiently strong blade cannot be created to resist the induced bending, without requiring a large section modulus. A large section modulus requires thick blade sections, typically 25% to 35% of the chord dimension, which results in excessive drag. The resultant CL/CD is typically below 44, yielding an aerodynamic efficiency of 42% to 48%. A high efficiency thin section blade, such as the NACA 6412, with a CL/CD of >110, cannot be used in wind turbines because of this strength requirement.
The mechanical efficiency (ηm) is primarily reflected in the turbine gearbox, required to convert the 16-25 RPM of the multi-bladed rotors to 1200 RPM and higher, in order to drive one to four generator assemblies. These high-ratio, multistage gearboxes are required to achieve the desired 50:1 to 75:1 speed increases. As each stage is only 98.5%±0.5% efficient, a four-stage gearbox would therefore have a maximum efficiency of 92% to 96%.
Finally, the electrical efficiency (ηe) consists of both the generator efficiency and the associated conversion process efficiency needed to achieve the requisite high voltage, 3-phase, 60 Hz power for grid compatibility. A typical high performance generator efficiency is between 88% to 92% for either AC or DC embodiments. With a transformer, for use with an AC generator, the efficiency is typically between 96.5% to 98.5% yielding a net overall average of 88%. With use of a DC generator, with an efficiency of 88% to 92% and a solid state inverter with an efficiency of 97% to 98%, the net overall average remains at 88%.
In summary: a blade efficiency of 45%, a gearbox efficiency of 96%, and a power generation and conversion efficiency of yields a net system overall efficiency of 38%, or (ηb)(ηm)(ηe)=ηo. A tabulation of the performance for these prior art designs would confirm this value for the net overall efficiency and show that, once the Betz Limit is included, the total specific energy extracted is approximately 22.5% of the theoretical wind energy.